Visible radiation sensitive compositions have been used for years in photopolymerizable compositions for the production of visible radiation sensitive materials, such as for example printing plates. In particular for novel techniques (i.e. exposure by means of lasers) an improved sensitivity is required, in particular within the visible range of the spectrum, in order to shorten the duration of the exposure. From an economical point of view, it is also important that low-intensity lasers can be used which are less expensive and more reliable than high-intensity lasers. Therefore, attempts have been made for quite some time to increase the sensitivity of visible radiation sensitive compositions which are to be used in photopolymerizable compositions.
It is known that the radical polymerization of ethylenically unsaturated compounds can be initiated by radiation with visible light in the presence of photoreducible dyes and reduction agents, such as for example amines (U.S. Pat. No. 3,097,096). EP-A-122 223 discloses photoinitiators and photopolymerizable compositions which contain metallocenes. By means of such metallocenes it was possible to increase the sensitivity of the photopolymerizable layer and to thus reduce the necessary duration of exposure and the required power of the radiation source. Efforts were also made to obtain improved results by using further modified metallocenes, for example in EP-A-401 165, U.S. Pat. No. 4,590,287, EP-A-255 486, EP-A-256 981 and U.S. Pat. No. 5,106,722.
DE-A-40 08 815 describes a photopolymerizable composition comprising a polymerical binder, a radically polymerizable compound with at least one polymerizable group and at least one photooxidizable group in the molecule as well as a metallocene as a photoinitiator.
In order to obtain a further increase in sensitivity, an attempt was made to use the metallocene together with a coinitiator. EP-B-269 573 for example discloses liquid mixtures of photoinitiators using solutions of titanocene compounds in liquid photoinitiators of the .alpha.-hydroxy and .alpha.-aminoacetophenone derivative type. DE-A-38 32 032 describes a photopolymerizable mixture comprising a polymeric binder, a radically polymerizable compound with at least one polymerizable group, a photoreducible dye and a metallocene as an initiator as well as a coinitiator. The coinitiator is a trihalogenemethyl compound cleavable by radiation, which is to be used for increasing the visible radiation sensitivity. Those compounds having a triazine ring in the basic structure which carries two trihalogenemethyl groups are preferred.
DE-A-40 13 358 describes a special process for the production of printing plates or photoresists wherein metallocenes are used as photoinitiators, which is aimed at the improvement of photosensitivity.
U.S. Pat. No. 3,717,558 relates to metallocenes of elements belonging to the subgroups of the periodic table in combination with a further photoinitiator having an activated halogene-containing group for use in photopolymerizable recording material. These initiator combinations, however, are very oxygen- and hydrolysis-sensitive and are therefore hardly suitable for the production of printing plates and photoresist materials.
It is also known to use a combination of special organometal compounds and onium salts in a hardening agent for polymerizable compositions (U.S. Pat. No. 5,086,086). In this connection, organometal compounds are used as metallocenes, whose essential feature is that they comprise at least one metal-metal-sigma-bond, i.e. that at least two transition metal atoms are present in a complex. The curing agents of U.S. Pat. No. 5,086,086 are not used in combination with dyes for light-induced polymerisation.
U.S. Pat. No. 4,971,892 discloses photopolymerizable compositions which are especially suitable for printing plates and which are supposed to show a high degree of sensitivity towards visible radiation. As an initiator system for the radical polymerisation, these photopolymerizable compositions comprise an initiator selected from diaryliodonium salts, halogenated triazines and triarylsulfonium salts as well as a specific merocyanine dye.
Even though progress has been made in increasing the visible radiation sensitivity of photopolymerizable compositions, there is still a demand for compositions with further improved properties, in particular with an increased photosensitivity.